The overlap is highest between schizophrenia and bipolar
disorder (15 percent), moderate between bipolar disorder and
depression and between attention-deficit/hyperactivity disorder
(ADHD) and depression (about 10 percent), and lowest between
schizophrenia and autism (3 percent).

Overall, common genetic variations accounted for 17 percent to
28 percent of risk for the five mental disorders, according to the
study published in the Aug. 11 issue of the journal
Nature Genetics, which the researchers say is the largest
genome-wide study of its kind.

The project involved more than 300 scientists at 80 research
centers in 20 countries and was supported by the U.S. National
Institute of Mental Health (NIMH).

The investigators analyzed the genomes of several thousand
people with the five mental disorders and people without the
disorders.

"Since our study only looked at common gene variants, the total genetic overlap between the disorders is likely higher," study co-leader Naomi Wray, of the University of Queensland in Australia, said in an NIMH news release. "Shared variants with smaller effects, rare variants, mutations, duplications, deletions, and gene-environment interactions also contribute to these illnesses."

The genetic overlap between schizophrenia and depression could
prove important in terms of research and diagnosing patients,
according to the study authors. They expected to see more overlap
between ADHD and autism, but said the moderate overlap between
schizophrenia and autism is consistent with emerging evidence.

"Such evidence quantifying shared genetic risk factors among traditional psychiatric diagnoses will help us move toward classification that will be more faithful to nature," said Bruce Cuthbert, director of the NIMH's adult translational research and treatment development division. Cuthbert is also the coordinator of an NIMH project that is working to develop a mental disorders classification system for research based more on underlying causes.

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